olsrd: improve respawn settings

olsrd: improve respawn settings

This increases the amount of seconds to wait before a service restart
attempt from 5 to 15 seconds and allows unlimited retries. Olsrd
sometimes crashes together with a network interface and 5 seconds can
be too short to bring back up a crashed network interface, which is
required for olsrd to restart. It also adds the ability to configure
the wait time (respawn_timeout) and the number of retries
(respawn_retry).

Signed-off-by: Tobias Schwarz <info@tobias-schwarz.com>